1. Field of the Invention
The present invention relates to an image encoding apparatus which is capable of reducing the amount of information contained in an input signal and capable of a prediction encoding of the input signal for producing a signal adapted to transmit an image through a communication line. The present invention also relates to an image decoding apparatus which is capable of reproducing the image transmitted through the communication line from the image encoding apparatus.
2. Description of the Related Art
As a recent provision and a spread of a digital transmission network, a progress in an image processing technique and a development of a high-speed digital signal processing technique demands for realizing the video communication service have risen as new communication service.
In the video communication services, Videophone service, TV conference service have been provided, and those services are advancing toward the implementation with an aid of a high-functional network like ISDN (Integrated Service Digital Network). For higher communication service, a new communication network such as B-ISDN (Broad band-Integrated Service Digital Network) is now under active consideration. In another field rather than the communication, a new system is expected for efficiently treating the image information (image media).
In general, the image information contains so much information that it requires a wide-band transmission network in case a video signal is directly transmitted through a communication line without changing the video signal to an adaptive form. Hence, the direct transmission of the video signal is not realistic. The video signal, however, includes redundant information. The use of redundancy makes it possible to reduce the amount of information contained in the video signal. Hence, the image encoding (compressing) technique is widely used for efficiently treating the video signal.
Such an image encoding system has been actively considered in the field of communication, in particular, the transmission of a video signal. It results in yielding various proposals. Of those proposed image encoding systems, the present inventors know that an inter-frame prediction with motion compensation orthogonal transform encoding system can provide the highest encoding efficiency and is often used in recent days.
FIG. 1 shows the principle on which the inter-frame prediction with motion compensation orthogonal transform encoding system operates. The input video signal contains time information, for example motion contained in the image and spatial information about the content of one image frame. Both pieces of information provide redundancies. Then, the inter-frame prediction with motion compensation section 20 serves to remove the time redundancy from the input video signal. The resulting signal is sent to an orthogonal transform encoding section 21 in which the spatial redundancy is removed from the received signal. The video signal encoded in the orthogonal transform encoding section 21 is locally decoded and output to a frame memory section 22 in which the locally decoded signal is stored. This stored signal is used for inter-frame prediction of a next image frame. That is, the inter-frame prediction with motion compensation section 20, the orthogonal transform encoding section 21, and the frame memory section 22 form a loop referred to as an encoding loop.
The inventors of the present invention know that an image encoding apparatus includes the inter-frame prediction with motion compensation orthogonal transform encoding system operating on the foregoing known motion as shonw in FIG. 2 which is composed of FIGS. 2A and 2B.
The above-mentioned known image encoding apparatus includes an inter-frame prediction with motion compensation section 23, a frame memory section 24, a difference operating section 25, a DCT section 26, a quantizing section 27, an encoding control section 28, a dequantizing section 29, an inverse DCT section 30, and an adder 31.
Those components compose a loop through which a video signal travels in the describing order. At first, the input video signal is sent to the inter-frame prediction with motion compensation section 23 which serves to encode the input video signal at each image frame. That is, the inter-frame prediction with motion compensation section 23 reads as a predicted value one previous decoded image frame from the frame memory section 24. Then, the difference operating section 25 serves to produce a prediction error signal by taking a difference between the input image frame and the predicted value read from the frame memory 24. The prediction error signal is sent from the difference operating section 25 to the DCT section 26 in which the DCT operation (discrete cosine transformation) is performed about the prediction error signal for transforming the signal into the DCT coefficient. The DCT operation is a kind of orthogonal transformation. The DCT section 26 sends out a DCT coefficient to the quantizing section 27 in which the DCT coefficient is quantized at a proper level, that is, the information is compressed. The quantizing section 27 sends the quantized output to the outside and to the dequantizing section 29 under a control of the encoding control section 28. The dequantizing section 29 serves to perform the dequantizing operation about the quantized signal for picking up the DCT coefficient. Then, the DCT coefficient is sent to the inverse DCT section 30 in which the inverse DCT operation is performed about the DCT coefficient for transforming the DCT coefficient into the prediction error signal. The prediction error signal is added to the prediction error read from the frame memory section 24 in the adder 31. Then, the adding result is stored in the frame memory section 24 and is used for inter-frame prediction of the next image frame.
The above-mentioned known image encoding apparatus is adapted to quantize the DCT coefficient under the proper error allowance for the purpose of reducing the amount of information. Hence, the amount of coding output and the quality of the encoded image are controlled depending on the change of a quantizing step size. The quantizing step size is adjusted on the desired amount of coding output.
As described above, the above-mentioned image encoding apparatus is arranged to quantize the DCT coefficient under certain error allowance for the purpose of reducing the amount of information. It means that the quality degradation of the encoded image results from the quantizing error. Further, the dequantizing section 29 and the inverse DCT section 30 serve to perform the dequantization and the inverse DCT operation about all the encoded output for the purpose of sending the locally decoded signal back to the encoding loop, respectively.
Hence, in a case that the above-mentioned image encoding apparatus is used for image transmission, the encoded output is required to be received on the destination. However, in a case that the information is lost on the communication line, a mismatch takes place between the locally decoded output and the decoded output on the destination. It results in malfunctioning the inter-frame prediction encoding, thereby remarkably degrading the quality of the decoded image.
Further, the above-mentioned image encoding apparatus is incapable of properly adapting to the change of the amount of information contained in the input video signal. That is, in a case that the input image contains an abruptly changing motion, the amount of information contained in the video signal is so abruptly increased that the encoding efficiency is made lower, thereby deteriorating the quality of the encoded image. It is thus difficult to keep the quality of the encoded image constant.